Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging and, more specifically, to a cooling system and method for a magnetic resonance imaging device.
Discussion of Art
Magnetic resonance imagine (MRI) machines work by generating a very strong magnetic field using a superconducting magnet which consists of many coils or windings of wires through which a current is passed. Creating a strong magnetic field is accomplished using superconductivity, which involves reducing the resistance in the current-carrying conductors to practically zero by cooling them to ultra-low temperatures below the superconducting limits This may be achieved by immersing the coils in a bath of liquid cryogen, such as liquid helium, and/or by circulating liquid cryogen within cooling loops adjacent to, or through, the coils, or by providing solid conductive pathways that allow to withdraw heat from the coils.
As will be readily appreciated, maintaining an ultra-low temperature in the coils is necessary for proper operation of the MRI machine. However, during ramp, considerable heat may be generated, mainly in leads, switches and heaters, which is transferred to and deposited in the cold mass assembly. During persistent operation, the heat is introduced either via solid conduction components, such as current leads, penetrations or suspension elements, or by radiation or residual gas conduction coming from components of cryostat assembly with higher temperature. The heat coming to the cold mass that contains the coils may lead to boil-off or evaporation of the cryogen, requiring replenishment.
Considerable research and development efforts have therefore been directed at minimizing the need to replenish the boiling cryogen. This has led to the use of cryogen gas recondensing systems that utilize a mechanical refrigerator or cryocooler, also known as a cold head, to cool the cryogen gas and recondense it back to liquid cryogen for reuse.
However, from time to time the cooling may be interrupted in the MRI devices located in clinical environment. That happens, for example, when it becomes necessary to remove the cryocooler for replacement and/or servicing. It is desirable to accomplish this without discontinuing superconducting operation of the magnet because of the time and expense resulting from relatively long “down-time” and subsequent ramping up period of bringing the magnet back to superconducting operation. Replacement of the cryocooler must therefore be effected in the period after a problem or service need is detected and before superconducting operation ceases.
Another condition when cooling power ceases to be provided to the superconducting magnet is a power outage. During the outage, the non-operating coldhead introduces thermal short that may input additional heat to the superconducting system.
This period after the cooling power is interrupted and before the superconducting operation ceases is known as the ride-through period, during which the final period of superconducting magnet operation and helium boil-off continues before quenching of the superconducting magnet. Indeed, for magnets with closed helium inventory, i.e., low cryogen type magnets, the duration of tolerable power outage, coldhead service or ramp profile is limited by the volume of accumulated liquid helium that boils off or evaporates during the above conditions with extra heat load.
It is therefore desirable to be able to extend the ride-through period to provide sufficient time for detection and correction of a problem such as by replacement of a cryocooler, to withstand a power outage, and also to avoid the possibility of peak temperatures being generated by superconducting operation quench which could exceed the critical temperature of the superconducting wires with which the magnet coils are wound.